(1) Field of the Invention
The present invention relates to propulsors, specifically to a linear actuator that produces oscillatory motion. The oscillatory motion is employed by flapping hydrofoils used in propulsors for undersea vehicles.
(2) Description of the Prior Art
It is known in the art that there are significant differences between heaving-pitching foil propulsion and conventional propulsion. The design of current underwater propulsors is based on steady-state hydrodynamic and aerodynamic theories as well as experimental knowledge. This is true of aircraft and undersea vehicles with this branch of engineering reaching a high level of maturity.
Further improvement in conventional propulsion will be incremental if the basic mechanism of production of lift on a hydrofoil remains largely the same. Conversely, if new and powerful mechanisms of lift production can be found and computational methods of hydrofoil blade design for implementing those mechanisms can be developed; new material technologies, control theories, and information processing architecture can be implemented.
For heaving-pitching foil propulsion, a flapping hydrofoil is used. In operation, the hydrofoil moves about an axis transverse to the direction of vehicle movement as does a rudder, but the hydrofoil oscillates so as to generate vortices about axes transverse to this direction. A single hydrofoil may be used or a plurality of hydrofoils variously moving toward or from each other may be used. The hydrofoil movements, and phases of multiple hydrofoils, may be variously intermittent, may be altered in frequency and amplitude, or may be asymmetric. These variations are advantageously selected for conditions when wake detection or reduction is not important, when a vehicle speed changes, or when the vehicle maneuvers.
Based on neural mechanics, a significant improvement in the development of quieter heaving-pitching propulsors is likely. Research into biology rather than physics indicates the feasibility that complex active systems can indeed be miniaturized and can be functional competitive.
Based on steady-state hydrodynamics and aerodynamics, flying insects like fruit flies are not supposed to fly; yet the insects do. It has been shown, using scaled up models of flying insects like fruit flies, that the fruit flies possess three mechanisms of lift enhancement. These lift mechanisms are based on unsteady hydrodynamics and not steady-state hydrodynamics.
First, the lift mechanisms produce vortices at the leading and trailing edges of the wings of the fruit flies. This dynamic stall delays conventional stall and allows higher levels of lift forces to be produced. Second, a rotational effect occurs due to wing rotation. It has also been shown that efficiency is highest and maximum lift is produced when the center of rotation is at about the quarter chord point from the leading edge. The third lift mechanism is wake or vortex capture.
As such, an improvement to propulsion would be to help apply the effects of the lift mechanisms, one or two or all three of the effects. The improved mechanisms could be used with undersea vehicles to enhance the lift produced by propulsion blades and the rotational speed (RPM=revolutions per minute) can thus be reduced.
As is also known in the art, there are three sources of propulsion radiated noise coming from a rotor blade. The first source of propulsion radiated noise is due to the ingestion of upstream vehicle turbulence by the rotor blade. The second source of propulsion radiated noise is blade tonals due to the gust created by a rotor blade shearing through the wake of the upstream stator blade. The third source of propulsion radiated noise is trailing edge vibration.
These three sources of propulsion radiated noise are proportional to the 4th, 5th and 6th power of RPM. When the RPM is reduced, the noise due to all these three sources, are reduced. In heaving and pitching propulsion, frequencies are 1/100th or even less than those in “conventional” propulsors.
As such, an improvement in decreasing radiated noise would be to go further than simply applying a heaving and pitching mechanism. One such improvement would be implementing the heaving and pitching mechanism in an even quieter manner by the use of an improved actuator.
Presently, the oscillatory motion of actuators is produced by servo-gear drives, which tend to have a modest efficiency. Thus, there is also a need for more efficient mechanisms for producing oscillatory motions in hydrofoils. More importantly, even apart from efficiency, servo-gear drives produce noise and vibration in the hull, which in turn radiates noise. As such, there is a need to lower such drive noise and vibration.